The invention relates to an improved acoustic baffle of controllable frequency characteristics for attentuation of acoustic energy impingent upon an underwater transducer of a sonic detecting system for minimizing the deleterious effect otherwise produced by energy emanating from the power plant of the vessel carryin the detection system. More particularly the inventive concept is directed to an improved baffle apparatus for attenuation of noise or undesired acoustic energy by utilization of a laminated construction of elements to provide predetermined attenuation characteristics while preserving the desirable features of prior art devices incorporating an externally pressurizable pneumatic rubber mat, which is attached to a steel backing plate.
The instant invention is an improvement over a pneumatic rubber mat device of an externally pressurizable character, disclosed in the co-pending application of Donald W. Kuester, Ser. No. 357,019, filed Mar. 31, 1964 for Air Compensated Underwater Sound Barrier. The acoustic performance of a baffle of the character of the aforementioned patent application has been found to be adequate over an acceptable band of frequencies. However, certain shortcomings have been experienced due to problems encountered with respect to unavailability of materials used possessing certain desired characteristics, i.e., those used proving to be mechanically unreliable under operational conditions, and further because of difficulties directed both to fabrication and quality control problems. All other known apparatus and prior art techniques directed to methods of attempting to achieve the purpose of the instant invention have been found to be acoustically inferior to the aforementioned Air Compensated Underwater Sound Barrier.
In its broadest sense, the present inventive concept is directed to the provision of improved apparatus for attenuating acoustic noise emanating from a sea going vessel, as propagated or transmitted through the surrounding water and which noise energy is of a character normally producing adverse and deleterious effects upon the performance of an acoustic transducer used in adjacency to the exterior of the ship hull. More particularly the invention relates to a acoustic baffle construction techniques and apparatus for effectively reducing sound transmission, from the hull of a ship, such for example as that produced by hull vibrations resulting from the ship's power plant and/or other propulsion noises, to an underwater sound transducer as installed on the vessel external to the hull thereof.
Vibrations of the hull arising from sources such as moving prime mover parts and both internal and external propulsion machinery of the ship is known to produce acoustic noise energy, which when transmitted through the water after transduction by the hull of the ship or by other of the aforementioned instrumentalities, increase the likelihood of detection by underwater listening Sonar equipment of an enemy, as well as affecting as by interference with the ship own Sonar equipment, the performance of transducers used for the ship's own Sonar and other types of detection equipment.
Hydrophone or underwater acoustic transducer devices used with the Sonar equipment of a ship, are limited in the effectiveness thereof in range and direction-sensitivity by noises produced by the ships propulsion machinery as transmitted to the hull, where they are transduced and propagated throughout the underwater environment ambient to the ship. This acoustic noise presents a serious problem to those vessels utilizing underwater acoustic equipment. Every effort to suppress or otherwise prevent the undesired background signals as produced by the ship machinery and propulsion equipment from being transmitted or coupled to the hydrophone or transducers mounted exterior of the surface of the ship warrants investigation.
Toward this end, underwater sound baffles have been developed for shielding the underwater transducers from noise emanating from the ship. The aforementioned Kuester et al application is an example of one such expedient. Other approaches to the solution of the problems have been characterized by the use of sound-absorbent or sound reflecting material, such for example as rubber fabricated in a honey-comb fashion with air pockets therein. Frequently these pockets, or voids, are filled with shot or the like to vary the acoustic impedance characteristics thereof. It has been found that this type of construction is not entirely satisfactory under all conditions of service, however, since the air pockets tend to collapse under high hydrostatic pressures. At depths beyond approximately one-hundred feet it has been found desirable to resort to other equipment to accomplish the desired purpose. Modern submarines operate at depths far exceeding one-hundred feet; accordingly, it is apparent that other underwater sound baffle structures capable of operating at unlimited depth must be provided if optimum performance is to be derived from the ship's Sonar and other acoustic detection equipment.
It is a feature of the instant invention to provide an improved structure untilizing a unique relationship of the elements or parts thereof, for attenuation of ambient noise energy emanating from the hull of a ship or the like, for improving environmental operational conditions of underwater acoustic transducers disposed in proximity to the hull of a ship, and for enhancement of the overall acoustical performance of the transducers.
One object of the instant invention resides in the provision of improved acoustic baffle structures of a charactor adapted for utilization with underwater acoustic transducers at deep operational depths for materially reducing or the substantially complete attenuating of noise transmitted through the water, by coupling effects of the ship's hull, in transducing random noise emanating from the ships propulsion equipment.
A further object of the invention resides in the provision of an improved underwater-sound baffle apparatus having a capability for variable pneumatic pressurization for reducing the transmission of noise from the hull of a ship to an underwater sound transducer mounted exteriorly of the ship and on the baffle.
Another object is to provide an improved underwater sound baffle apparatus presenting frequency selective sound attentuation characteristics, rendering the baffle adaptable to operation at a variety of water depths for attenuation of acoustic energy radiated from a sound source emanating within the hull of a ship and transduced by the ship's hull to the water in which a hydrophone or acoustic transducer is mounted relative to the baffle.
Other objects and attendant advantages of the instant invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein: